Reduced gain variation biasing for short channel devices

ABSTRACT

An amplifier biasing circuit that reduces gain variation in short channel amplifiers, an amplifier biasing circuit that produces a constant Gm biasing signal for short channel amplifiers, and a multistage amplifier that advantageously incorporates embodiment of both types of amplifier biasing circuits are described. Both amplifier biasing circuit approaches use an operational amplifier to equalize internal bias circuit voltages. The constant Gm biasing circuit produces a Gm of 1/R, where R is the value of a variable resistor value. The biasing circuit that reduces gain variation produces a Gm of approximately 1/R, where R is the value of a variable resistor value, however, the biasing circuit is configurable to adjust the bias circuit Gm to mitigate the impact of a wide range of circuit specific characteristics and a wide range of changes in the operational environment in which the circuit can be used, such as changes in temperature.

INCORPORATION BY REFERENCE

This present disclosure is a continuation of U.S. application Ser. No.13/462,968, filed on May 3, 2012, which is a continuation of U.S.application Ser. No. 13/193,998, filed on Jul. 29, 2011, which is acontinuation of U.S. application Ser. No. 12/606,746, filed on Oct. 27,2009, which claims priority under 35 U.S.C. §119(e) to U.S. ProvisionalApplication No. 61/149,444, filed on Feb. 3, 2009, and U.S. ProvisionalApplication No. 61/112,003, filed on Nov. 6, 2008.

BACKGROUND

The physical dimensions of transistors used to construct an integratedcircuit determine the maximum frequency at which the integrated circuitcan be operated. In general, the smaller the feature dimensions of atransistor, e.g., the gate dimensions, the higher the maximum frequencyat which the transistor can be operated. For example, an exampleembodiment of a high frequency transistor, or short channel transistor,that is suitable for use in the Gm stage of a 5.5 GHz amplifier may havea channel length of approximately 60 nanometers (nm).

The gain response produced by a high frequency amplification circuitdesign that uses short channel transistors can vary when the sameamplifier circuit design is implemented using different transistortechnologies, i.e., process changes. Further, the gain response producedby a high frequency amplification circuit design that uses short channeltransistors can vary in response to changes in the operatingenvironment, e.g., the operating temperature, of the amplificationcircuit. In addition, the gain response produced by a high frequencyamplification circuit that uses short channel transistors can beadversely affected by small manufacturing variances in the physicaldimensions, e.g., physical gate dimensions, of transistors used in thecircuit.

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

SUMMARY

A short channel amplifier biasing circuit that reduces gain variation inhigh frequency amplification circuits, a short channel amplifier biasingcircuit that produces a constant Gm biasing signal for high frequencyamplification circuits, and a multistage amplifier that advantageouslyincorporates embodiments of both types of short channel amplifierbiasing circuits are described. Both amplifier biasing circuitapproaches use an operational amplifier to equalize internal biascircuit voltages. The constant Gm biasing circuit produces a Gm of 1/R,where R is the resistance value assigned to a circuit trim resistor. Thebiasing circuit that reduces gain variation produces a Gm ofapproximately 1/R, where R is the resistance value assigned to a circuittrim resistor, however, the biasing circuit is configurable to adjustthe bias circuit's Gm to mitigate the impact circuit-specific variationsand to mitigate the impact of variations in the operational environmentin which the circuit can be used, such as variations in operatingtemperature.

Bias circuits using current mirror configurations that operate wellusing low frequency, long channel transistors, do not perform well whenusing high frequency, short channel transistors. One reason for thispoor performance is that the drain-to-source voltage, Vds, of thedriving transistor device in the biasing circuit is different from thedrain-to-source voltage, Vds, of the transistor device receiving abiasing signal generated by the biasing circuit. The described biasingcircuits overcome this problem by using an operational amplifier toequalize the respective drain-to-source voltages, thereby allowing thecurrent mirror biasing circuits to perform well using high frequency,short channel transistors.

The described biasing circuits may be used in a wide range ofapplications, including semiconductor integrated circuit systems on achip that include control interfaces to external devices. For example,the described biasing devices may be used by the pre-power amplifier ina controller for an external power amplifier used to power a radiofrequency antenna, or may be used in a low noise amplifier used toamplify a signal received via a radio frequency receiver. However, thedescribed biasing circuits may be used in any circuit that requiresbiasing circuits that can be adapted to mitigate the varied effects ofprocess change and operating conditions on an amplifier circuit.

In one example embodiment, a bias signal circuit is described thatincludes, a first current branch that includes, a first cascadetransistor, and a constant current source connected in series with thefirst cascode transistor, a second current branch that includes, asecond cascode transistor with a channel width-to-length ratio that isapproximately 4 times a channel width-to-length ratio of the firstcascode transistor, the gate of the first cascade transistor and secondcascade transistor connected to a common node, a short channel mirrordevice transistor connected in series with the second cascadetransistor, and a trim resistor connected in series between the secondcascode transistor and the short channel mirror device transistor, andan operational amplifier, a first input node of the operationalamplifier connected between the trim resistor and the short channelmirror device transistor at a drain terminal of the short channel mirrordevice transistor, a second input node of the operational amplifierconnected between the first cascade transistor and the constant currentsource, and an output node connected to a gate terminal of the shortchannel mirror device transistor.

In a second example embodiment, a bias signal circuit is described thatincludes, a first current branch that includes a first cascadetransistor, and a first short channel transistor connected in serieswith the first cascade transistor, a second current branch that includesa second cascade transistor, a gate of the first cascade transistor anda gate of the second cascade transistor connected to a common node, asecond short channel transistor, with a channel width-to-length ratiothat is approximately 4 times the channel width-to-length ratio of thefirst short channel transistor, connected in series with the secondcascode transistor, and a trim resistor connected in series with thesecond cascode transistor and the second short channel transistorbetween a source terminal of the second short channel transistor and aLOW source voltage, and an operational amplifier, a first input node ofthe operational amplifier connected between the first cascode transistorand a drain terminal of the first short channel transistor, a secondinput node of the operational amplifier connected between the secondcascade transistor and a drain terminal of the second short channeltransistor, and an output node of the operational amplified connected toa gate terminal of the first short channel transistor and a gateterminal of the second short channel transistor.

In a third example embodiment, a multistage amplifier is described thatincludes a first amplifier stage that receives a bias signal generate bythe bias circuit of the first example embodiment described above, and asecond amplifier stage that receives a bias signal generate by the biascircuit the second example embodiment described above, in which an inputsignal provided to the second amplifier stage amplifies is an outputsignal generated by the first amplifier circuit.

In a fourth example embodiment, a method of controlling a variablecurrent source is described that includes, monitoring a magnitude of afirst monitored current passing through a mirror transistor device of anamplifier biasing circuit, and adjusting a magnitude of a first drawncurrent drawn away from the mirror transistor device through thevariable current source based on the magnitude of the first monitoredcurrent and a first monitored current first setting.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of an amplifier biasing circuit that reduces gain variation inshort channel amplifiers, examples of an amplifier biasing circuit thatproduces a constant Gm biasing signal for short channel amplifiers, andexamples of a multistage amplifier that uses example embodiments of bothtypes of biasing circuits, will be described with reference to thefollowing drawings, wherein like numerals designate like elements.

FIG. 1 is a schematic diagram of an example embodiment of an amplifierbiasing circuit that reduces gain variation in short channel amplifiers.

FIG. 2 is a more detailed schematic diagram of the amplifier biasingcircuit shown in FIG. 1.

FIG. 3 is a more detailed schematic diagram of the Vdsat current sourceshown in FIG. 2.

FIG. 4 is a block diagram of a multi-stage amplifier in which the biassignal provided to the first amplifier stage is produced by an exampleembodiment of the biasing circuit shown in FIG. 1, and in which the biassignal provided to the second amplifier stage is produced by an exampleembodiment of the biasing circuit shown in FIG. 10.

FIG. 5 a flow-chart of a process for starting up an example embodimentof the amplifier biasing circuit shown in FIG. 1.

FIG. 6 a flow-chart of a process that may be used by a technician toconfigure an example embodiment of the amplifier biasing circuit shownin FIG. 1 with a baseline constant current and a tuned resistor.

FIG. 7 a flow-chart of a process for adjusting a current level in anexample embodiment of the amplifier biasing circuit shown in FIG. 1 inresponse to input from a monitoring sensor.

FIG. 8 and FIG. 9 are a flow-chart of a process that may be used by atechnician to tune an example embodiment of the amplifier biasingcircuit shown in FIG. 1.

FIG. 10 is a schematic diagram of an example embodiment of an amplifierbiasing circuit that produces a constant Gm biasing signal for shortchannel amplifiers.

FIG. 11 is a more detailed schematic diagram of the amplifier biasingcircuit shown in FIG. 10.

FIG. 12 a flow-chart of a process for starting up an example embodimentof the amplifier biasing circuit shown in FIG. 10.

FIG. 13 is a flow-chart of a process for configuring an exampleembodiment of the amplifier biasing circuit shown in FIG. 10 with anequivalent Gm device channel width to channel length ratio (W/L) and atuned resistor.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic diagram of an example embodiment of an amplifierbiasing circuit that reduces gain variation in short channel amplifiers.As shown in FIG. 1, biasing circuit 100 includes a first cascodetransistor 102, a second cascode transistor 104, a trim resistor 106, amirror device transistor 110, an operational amplifier 112, a variablecurrent source 114, and a constant current source 116. First cascodetransistor 102 includes a gate terminal, a source terminal and a drainterminal. The channel length of the gate channel of first cascodetransistor 102 may be the same as the channel length of the gate channelof second cascode transistor 104; however, the channel width to channellength ratio (W/L) of the gate channel of first cascode transistor 102is greater, e.g., approximately 4 times greater, than the channel widthto channel length ratio (W/L) of the gate channel of second cascodetransistor 104. Second cascode transistor 104 includes a gate terminal,a source terminal and a drain terminal. Trim resistor 106 includes acurrent path first terminal, a current path second terminal and a trimcontrol terminal 108. Mirror device transistor 110 includes a gateterminal, a source terminal and a drain terminal. Operational amplifier112 includes a non-inverted input terminal, a inverted input terminaland an output terminal. Variable current source 114 includes a firstterminal and a second terminal. Constant current source 116 includes afirst terminal and a second terminal.

As further shown in FIG. 1, the drain terminal of cascode transistor 102is connected to HIGH voltage source V_(DD), the source terminal ofcascode transistor 102 is connected to the current path first terminalof trim resistor 106, and the gate terminal of cascade transistor 102 isattached to the gate terminal of cascade transistor 104 at node 120,which may be connected to a threshold bias voltage source V_(COS). Thedrain terminal of cascode transistor 104 is connected to HIGH voltagesource V_(DD), and the source terminal of cascade transistor 104 isconnected to the inverted input terminal of operational amplifier 112and the first terminal of constant current source 116 at node 122. Thecurrent path second terminal of trim resistor 106 is connected to thedrain terminal of mirror device transistor 110, the non-inverted inputterminal of operational amplifier 112, and the first terminal ofvariable current source 114 at node 118. The source terminal of mirrordevice transistor 110 is connected to LOW power source, V_(SS), and thegate of mirror device transistor 110 is connected to the output terminalof operational amplifier 112 at node 124. The second terminal ofconstant current source 116 and the second terminal of variable currentsource 114 are connected to LOW power source, V_(SS). In addition, thesilicon substrate of cascade transistor 102, cascade transistor 104, andmirror device transistor 110 are connected to low voltage source,V_(SS).

During operation, as process based circuit component characteristicsand/or operating temperature causes the Gm of biasing circuit 100 todecrease, the gate-to-source voltage, Vgs, of cascode transistor 104increases, since cascode transistor 104 sources a constant current. Inresponse, operational amplifier 112 adjusts so that the gate-to-sourcevoltage, Vgs, of cascade transistor 102 plus the voltage drop acrosstrim resistor 106 increases by the same amount as the gate-to-sourcevoltage, Vgs, of cascade transistor 104. Since the width to length ratioof cascade transistor 102 is larger than the width to length ratio ofcascode transistor 104 the gate-to-source voltage, Vgs, of cascodetransistor 102 will increase some, but not as much as the gate-to-sourcevoltage, Vgs, of cascade transistor 104 due to the reduced currentdensity in cascode transistor 102. Therefore, a portion of the increasedvoltage will be across trim resistor 106, thus, the bias current throughtrim resistor 106 will increase. The bias current is increased byincreasing the gate voltage of mirror device transistor 110. Thisincreases the bias point and the Gm of biasing circuit 100. Thus, thecircuit adjusts to counteract changes in the Gm of the circuit due to,for example, circuit component performance characteristics at differentoperating temperature.

Unlike a true constant Gm circuit, as described below with respect toFIG. 10 and FIG. 11, biasing circuit 100 just pushes Gm back in theright direction rather than keeping the Gm exactly the same. Further, asdescribed in greater detail below, biasing circuit 100 may include avariable current source that can add or remove current to/from thecurrent coming down to mirror device transistor 110, thus addingadditional control to the bias point. The drain-to-source voltage, Vds,of mirror device transistor 110 is controlled by the gate-to-sourcevoltage, Vgs, of cascade transistor 104. As long as the fixed currentproduced by constant current source 116 is close to the bias currentthrough mirror device transistor 110, the drain-to-source voltage ofmirror device transistor 110 will be close enough to the drain-to-sourcevoltage of the amplifier stage receiving the biasing signal generated bybiasing circuit 100 to avoid mismatch due to short channel effects.

Bias circuit 100 reduces Gm variation, but does not set Gm exactly. Biascircuit 100 has a single feedback loop and a single stable point thatdoes not require a start up circuit. Further, additional currents can beintroduced without causing startup issues. The ability to add additionalcurrents allows bias circuit 100 to account for other sources of gainvariation. Bias circuit 100 can, in addition to changing the sizes ofdevices and the resistance value assigned to trim resistor 106, programvariable and fixed currents which can be used to change the bias pointand how bias circuit 100 reacts to changes in operating temperature. Inone example embodiment, bias circuit 100 keeps the drain-to-sourcevoltage, Vds, of the mirror device close to the drain-to-source voltage,Vds, of the gain device of the amplifier stage receiving a bias signalfrom bias circuit 100. This approach keeps mismatch due to short channeleffects minimal but does not eliminate them. If a true constant Gmbiasing signal is required, a biasing circuit, such as biasing circuit1000, described with respect to FIG. 10 may be used.

FIG. 2 is a schematic diagram of the amplifier biasing circuit describedabove with respect to FIG. 1, in which additional details are shown.Features described above with respect to FIG. 1 are identified withcorresponding numeric labels. For example, first cascode transistor 102,second cascode transistor 104, trim resistor 106, mirror devicetransistor 110, operational amplifier 112, and constant current source116 are depicted in a manner similar to that shown in FIG. 1, andtherefore, are not further described. However, as shown in FIG. 2,biasing circuit 100 can include additional features such as HIGH sourcevoltage filter/power-down circuit 202, a power-down switch 206, acascode gate storage capacitor 208, a bias point low-pass filter 210,and an optional current tuning circuit 212. Further, as shown in FIG. 2,current source 114 can include additional features such as, adigital-to-analog converter (DAC) current source 214, and a Vdsatcurrent source 216.

HIGH source voltage filter/power-down circuit 202 filters power receivedby biasing circuit 100 from HIGH source voltage, V_(DD), and allowsbiasing circuit 100 to be disconnected from HIGH source voltage tosupport circuit power-down operations. Power-down circuit 206 allows thegates of the cascode transistor 102 and cascode transistor 104 to pulledHIGH or LOW, as needed, to support circuit configuration and/or circuitpower-down operations. Cascode gate storage capacitor 208 is used duringoperation to maintain a gate voltage established across the gates ofcascode transistor 102 and cascode transistor 104. Bias point low-passfilter 210 allows biasing circuit 100 to control the startup time of theamplifier.

Optional current tuning circuit 212 allows biasing circuit 100 to bemanually configured as a constant gain biasing circuit that produces aconstant Gm biasing signal for short channel amplifiers. For example,optional current tuning circuit 212 may be used to configure biasingcircuit 100 to provide a constant gain biasing signal to an amplifiercircuit in which gain variation is not a concern. In such an embodiment,current source 114 is disabled.

IDAC current source 214 receives a digital signal from one or moresensors, e.g., one or more temperature sensors, and adjusts a currentapplied to biasing circuit 100 at node 118 based on the magnitude of theone or more sensor values received. For example, in one exampleembodiment, IDAC current source 214 can adjust the amount of currentsupplied to biasing circuit 100 at node 118 whenever a change in themagnitude of any one of the one or more sensor values is detected.

Controlling a temperature based current can be performed by an IDACcurrent source 214 acting alone, or in combination with a Vdsat currentsource 216, as described below with respect to FIG. 3. If there isalready an on chip temperature sensor that produces a digital code, thenthat digital code can be used to control IDAC current source 214.Digital logic may be used to convert a received digital temperature to acontrol code that can be used to control an amount of temperaturedependent current output from IDAC current source 214. In one exampleembodiment, when used in combination with a Vdsat current source 216,the amplitude of the current produced by Vdsat current source 216 isdynamically chosen to adjust for process variation and current outputfrom IDAC current source 214 is dynamically chosen to adjust for anyadditional temperature variation.

Vdsat current source 216 is configured to sink current from biasingcircuit 100 at node 118 based on a sensed magnitude of the currentsupplied to biasing circuit 100 at node 118 from IDAC current source 214and/or the current supplied to biasing circuit 100 at node 118 via trimresistor 106. For example, since the Gm of the amplifier may change inresponse to changes in operating conditions, e.g., changes intemperature, Vdsat current source 216 may be configured to compensatefor such changes in the Gm of the amplifier by pulling current away frommirror device 110, as described below with respect to FIG. 8 and FIG. 9.

FIG. 3 is a schematic diagram of Vdsat current source 216 describedabove with respect to FIG. 1 and FIG. 2, in which additional details areshown. Vdsat current source 216 monitors a current output from IDACcurrent source 214 to node 118 of biasing circuit 100 and monitors acurrent, or poly-current, received at node 118 from trim resistor 106and draws current away from node 118 of biasing circuit 100, i.e., awayfrom mirror device transistor 110, based on one or more of the monitoredcurrents. For example, Vdsat current source 216 can be used to adjustthe gain of a first amplifier stage to adjust for changes in a gainapplied by a second amplifier stage, as described below with respect toFIG. 4.

As shown in FIG. 3, Vdsat current source 216 can includeconnect/disconnect switch 302, IDAC current over-bias control 304,poly-current over-bias control 306, current mirror 308, thresholdvoltage circuit 310, resistor 312, first poly-current control 314, firstIDAC current control 316, first current mirror 318, second poly-currentcontrol 320, second IDAC current control 322, and second current mirror324. Connect/disconnect switch 302, is connected between node 342 andnode 334. IDAC current over-bias control 304 is connected between node332 and node 330. Poly-current over-bias control 306 is connectedbetween node 334 and node 330. Current mirror 308 is connected betweennode 332 and node 330. Resistor 312 is connected between node 334 andone of a source and a drain of threshold voltage circuit 310. The otherof the source and drain of threshold voltage circuit 310 is connected tonode 330. First poly-current control 314 is connected between node 334and a gate of first current mirror 318. First IDAC current control 316is connected between node 332 and the gate of first current mirror 318.First current mirror 318 is connected between node 340 and node 330.Second poly-current control 320 is connected between node 334 and a gateof second current mirror 324. Second IDAC current control 322 isconnected between node 332 and the gate of second current mirror 324.Second current mirror 324 is connected between node 340 and node 330.

Vdsat current source 216 monitors the current supplied by IDAC currentsource 214 to node 118 of bias control circuit and optionally monitorsthe poly-current that passes through trim resistor 106 to node 118. Themagnitude of the respective monitored currents is used to determine theamount of current that Vdsat current source 216 pulls from node 118.

For example, prior to operation, Vdsat current source 216 can beconfigured by a technician using the process flow described below withrespect to FIG. 8 and FIG. 9. For example, Vdsat current source 216 isconfigured to monitor the poly-current through trim resistor 106, inaddition to the IDAC current, by configuring the control leads connectedto the gates of transistors in connect/disconnect switch 302 with binarysignals that close connect/disconnect switch 302.

First current mirror 318 may be configured to mirror the IDAC current byapplying binary control signals to first IDAC current control 316 andfirst poly-current control 314 that CLOSE first IDAC current control 316and OPEN first poly-current control 314. First current mirror 318 may beconfigured to mirror the poly-current by applying binary control signalsto first IDAC current control 316 and first poly-current control 314that OPEN first IDAC current control 316 and CLOSE first poly-currentcontrol 314. Second current mirror 324 may be configured to mirror theIDAC current by applying binary control signals to second IDAC currentcontrol 322 and second poly-current control 320 that CLOSE second IDACcurrent control 322 and OPEN second poly-current control 320. Secondcurrent mirror 324 may be configured to mirror the poly-current byapplying binary control signals to second IDAC current control 322 andsecond poly-current control 320 that OPEN second IDAC current control322 and CLOSE second poly-current control 320.

To disable Vdsat current source 216 so that no current is pulled, binarycontrol signals are applied to IDAC current over-bias control transistor304 and IDAC current over-bias control transistor 306 to CLOSE bothtransistors. To enable Vdsat current source 216 so that current can bepulled, binary control signals are applied to IDAC current over-biascontrol transistor 304 and IDAC current over-bias control transistor 306to OPEN both transistors. When Vdsat current source 216 is enabled, thedynamic current pulled through Vdsat current source 216 is determined bythe combinations of binary control signals applied to first IDAC currentcontrol 316, first poly-current control 314, second IDAC current control322 and second poly-current control 320, as described above.

Removing current from node 118 going to mirror device transistor 110causes the bias point of biasing circuit 110 to move further in reactionto process and temperature. For example, the Gm of the mirror devicewill increase as temperature increases. Vdsat current source 216 isprogrammable to blend Vdsat current and constant current from IDACcurrent source 118, described above. This allows the degree of reactionto process and temperature to be programmed.

FIG. 4 is a block diagram of a multi-stage amplifier in which the biassignal provided to the first amplifier stage is produced by an exampleembodiment of biasing circuit 100, described above with respect to FIG.1 through FIG. 3, and in which the bias signal provided to the secondamplifier stage is produced by an example embodiment of the biasingcircuit described below with respect to FIG. 10 and FIG. 11. As shown inFIG. 4, an example embodiment of multi-stage amplifier 400 includes afirst amplifier bias circuit 402, first amplifier stage 404, secondamplifier bias circuit 406 and second amplifier stage 408. Multi-stageamplifier 400 may be a high frequency amplifier, such as a radiofrequency signal amplifier. To allow multi-stage amplifier 400 toamplify high frequency signals, each amplifier stage in multi-stageamplifier 400 may include short channel transistor devices, e.g.,transistors in which the length of the transistor gate channel isapproximately 60 nanometers (nm).

For example, in one example embodiment, first amplifier stage 404 may bea high frequency amplifier that includes a short channel transistordevice as the Gm stage transistor of the amplifier. A bias signalproduced by first amplifier bias circuit 402 as well as a capacitivelycoupled high frequency input signal may be applied to the gate terminalof the first amplifier stage 404. The bias signal biases the firstamplifier stage 404 so that the amplifier can apply a gain to thereceived capacitively coupled high frequency signal. Further, secondamplifier stage 408 may be a high frequency amplifier that includes ashort channel transistor device as the Gm stage transistor of theamplifier. The capacitively coupled high frequency amplified outputsignal produced by first amplifier stage 404 as well as a bias signalproduced by second amplifier bias circuit 402 are applied to the inputgate terminal of second amplifier stage 408. The bias signal biases thesecond amplifier stage 408 so that the amplifier can apply a gain to thereceived capacitively coupled high frequency signal. As a result, theoutput of multi-stage amplifier 400 is the input signal received byfirst amplifier stage 404 times the gain of first amplifier stage 404times the gain of second amplifier stage 408.

In one example embodiment of multi-stage amplifier 400, first amplifierbias circuit 402 may be an amplifier bias circuit that reduces gainvariation in short channel amplifiers such as that described above withrespect to FIG. 1 through FIG. 3, and second amplifier bias circuit 406may be an amplifier that produces a constant Gm biasing signal for shortchannel amplifiers such as that described below with respect to FIG. 10and FIG. 11.

It is noted that the two stage, multi-stage amplifier described abovewith respect to FIG. 4 is exemplary only. Other example multi-stageamplifier embodiments may have more than two amplifier stages. Dependingon the desired characteristics of the final multi-stage amplifier outputsignal, and the characteristics of the respective amplifier stages underthe operating conditions in which the multi-stage amplifier is used, thebiasing circuits used to supply a biasing signal to the respectiveamplifier stages may be a biasing circuit that reduces gain variation inshort channel amplifiers, as described above with respect to FIG. 1through FIG. 3, or a biasing circuit that produces a constant Gm biasingsignal for short channel amplifiers, as described below with respect toFIG. 10 and FIG. 11, or some other bias circuit, such as a constantcurrent bias.

For example, one or more amplifier stages within the multistageamplifier may be biased using an embodiment of the biasing circuitdescribed below with respect to FIG. 1 through FIG. 3, in which therespective biasing circuits are configured to over compensate forprocess and temperature variations in subsequent amplifier stages thatcan affect the performance of the multistage amplifier, as describedbelow. One or more other amplifier stages may be biased for constant Gmusing an embodiment of the biasing circuit such as biasing circuitdescribed below with respect to FIG. 10 and FIG. 11. The number ofamplifier stages, the type of biasing circuit used to bias each, and themanner in which each biasing circuit is configured, may be driven by theamplification requirements to be met by the multi-stage amplifier andthe operational environment in which the multi-stage will be used.

For example, by adjusting the current put into bias circuit 100 at node118 based on a measured temperature, IDAC current source 214 maycompensate for changes in performance of components within the same orsubsequent amplifier stages, such as inductors and other components,that are temperature sensitive. Further, by adjusting the current pulledfrom bias circuit 100 at node 118 based on one or both of the IDACsupplied current supplied to node 118 and the poly-current supplied tonode 118 of bias circuit 100 via trim resistor 106, Vdsat current source216 of first amplifier bias circuit 402 may be configured to compensatefor variations in the Gm of the same or subsequent amplifier stages.

FIG. 5 a flow-chart of a process for starting up an example embodimentof the amplifier biasing circuit shown in FIG. 1 through FIG. 3. Asshown in FIG. 5, operation of the process begins at S502 and proceeds toS504.

At S504, power is supplied to bias circuit 100, and operation of theprocess continues at S506.

At S506, constant current source 116 may be activated to supply aconstant current through a first branch of bias circuit 100 thatincludes cascode transistor 104 and constant current source 116, andoperation of the process continues at S508.

At S508, the current established by constant current source 116establishes a gate-source voltage across the gate of cascode transistor104, thereby closing cascode transistor 104, and operation of theprocess continues at S510.

At S510, the voltage established across the gate of cascode transistor104 at S508, results in a voltage at node 120 and establishes agate-source voltage across the gate of cascode transistor 102, therebyclosing cascode transistor 102, and operation of the process continuesat S512.

At S512, an initial voltage is established at node 122 on the invertedinput node of operational amplifier 112, and operation of the processcontinues at S514.

At S514, a voltage difference between the voltages applied at the inputnodes of operational amplifier 112 activates operational amplifier 112,and operation of the process continues at S516.

At S516, mirror device transistor 110 is closed by the bias voltagegenerated at the output node of operational amplifier 112 at node 124and operation of the process continues at S518.

At S518, a current is allowed to pass through trim resistor 106 toestablish a voltage at node 118 and the non-inverted input node ofoperational amplifier 112, and operation of the process continues atS520.

At S520, if operational amplifier 112 determines that a differencebetween the voltage at node 118 and the voltage at node 122 is zero,operation of the process terminates at S524; otherwise, operation of theprocess continues at S522.

At S522, operational amplifier 112 adjusts the output of operationalamplifier 112 at node 124 based on the difference between the voltagesapplied to the two respective input nodes of operational amplifier 112at node 118 and at node 122, and operation of the process continues atS520.

At S520 and S522, above, a feedback loop is established between node 124and node 118 that includes operational amplifier 112 and mirror devicetransistor 110. Operational amplifier 112 continues to adjust thevoltage at node 124, until the input voltage at node 118 is again equalto the input voltage at node 122. Once the difference between the inputvoltage at node 118 and the input voltage at node 122 is zero, the biascircuit initialization process is complete.

It is noted that in the startup process for biasing circuit 100, thevoltage at node 122 is the high voltage supply, V_(DD), minus thegate-to-source voltage, Vgs, of cascode transistor 104 based on theconstant current produced by constant current source 116. There is nofeedback in this half of the circuit. The only feedback loop is thefeedback loop that contains operational amplifier 112, node 124, mirrordevice transistor 110, and node 118 which is connected to thenon-inverting input terminal of operational amplifier 112. Since thereis only the one negative feedback loop startup and stability are easy toanalyze. Startup does not depend on the current being added to, orsubtracted from node 118 by variable current source 114. If the outputof operational amplifier 112 at node 124 is high, the non-invertinginput will be low, causing the output to come down. If the output ofoperational amplifier 112 at node 124 is low, the non-inverting inputwill be high, causing the output to come up.

FIG. 6 is a flow-chart of a manual process that may be performed by atechnician to determine an appropriate resistor value for trim resistor106 within a specific implementation of bias circuit 100, e.g., animplementation of bias circuit 100 within a two-stage amplifier, such asmulti-stage amplifier 400 described above with respect to FIG. 4. Oncethe trim value is determined using the process shown in FIG. 6, the sametrim value may be used to configure similarly configured circuits. Asshown in FIG. 6, operation of the process begins at S602 and proceeds toS604.

At S604, bias circuit 100 is started as described above with respect toFIG. 5, and operation of the process continues at S606.

At S606, bias circuit 100 is allowed to stabilize, and operation of theprocess continues at S608.

At S608, IDAC current source 214 receives a measure of a physicaloperational characteristic of bias circuit 100 and/or an amplifier stageoperational characteristic, e.g., such as a temperature, from a sensor,e.g., a digital sensor, that provides input to IDAC current source 214,and operation of the process continues at S610.

At S610, IDAC current source 214 sets a magnitude of an IDAC currentsupplied by IDAC current source 214 to node 118 of biasing circuit 100.The magnitude of the IDAC current is determined based on the value ofthe measurement received at S608 and stored data, e.g., lookup tablescontaining previously determined current values for respective sensormeasurement values, and operation of the process continues at S612.

At S612, a technician determines a magnitude of the applied IDAC currentand the poly-current received at node 118 of bias circuit 100, anddecides whether the current levels are acceptable. If the current levelsare acceptable, operation of the process continues at S616; otherwise,operation of the process continues at S614.

At S614, the technician increments or decrements the resistor value,i.e., trims the resistor value, assigned to trim resistor 106, andoperation of the process continues at S606.

At S616, the technician permanently assigns the determined resistorvalue to trim resistor 106, and operation of the process terminates atS618.

FIG. 7 a flow-chart of a process used by an example embodiment ofbiasing circuit 100 to adjust the magnitude of the current applied tobias circuit 100 at node 118 by IDAC current source 214. The process isperformed automatically by biasing circuit and assumes that the value oftrim resistor 106 has already been determined and applied to trimresistor 106, as described above with respect to FIG. 6. As shown inFIG. 7, operation of the process begins at S702 and proceeds to S704.

At S704, bias circuit 100 is started as described above with respect toFIG. 5, and operation of the process continues at S706.

At S706, bias circuit 100 is allowed to stabilize, and operation of theprocess continues at S708.

At S708, IDAC current source 214 receives a measure of a physicaloperational characteristic of bias circuit 100 and/or an amplifier stageoperational characteristic, e.g., such as a temperature, from a sensor,e.g., a digital sensor, that provides input to IDAC current source 214,and operation of the process continues at S710.

At S710, IDAC current source 214 sets a magnitude of an IDAC currentsupplied by IDAC current source 214 to node 118 of biasing circuit 100.The magnitude of the IDAC current is determined based on the value ofthe measurement received at S708 and stored data, e.g., lookup tablescontaining previously determined current values for respective sensormeasurement values, and operation of the process continues at S712.

At S712, if a power-down condition is detected, operation of the processterminates at S712, otherwise; operation of the process continues atS706.

FIG. 8 and FIG. 9 are a flow-chart of a manual process that may be usedby a technician to tune an example embodiment of bias circuit 100 thatincludes both an IDAC current source 214 and a Vdsat current source 216,such as the IDAC current source 214 and the Vdsat current source 216described above with respect to FIG. 2 and FIG. 3. The process belowassumes that the example bias circuit 100 has been included in anamplifier circuit, such as amplifier circuit 400 described above withrespect to FIG. 4. In one example embodiment, once the respectivecontrol values are determined, the control parameters may be stored asdata that may be applied to Vdsat current source 216 and/or IDAC currentsource 214 at runtime. In another example embodiment, once therespective control values have been determined, only the controlparameters for the IDAC current source 214 are stored as data. The Vdsatcurrent source 216 is modified to permanently apply the determinedcontrol values. Once the Vdsat control parameters have been determinedfor a specific amplifier circuit embodiment, the same control parameterscan be applied to similarly configured circuits. In the processdescribed below, it is assumed that the resistor value for trimtransistor 106 has not yet been determined, i.e., the process describedabove with respect to FIG. 6 has not been implemented. As shown in FIG.8, operation of the process begins at S802 and proceeds to S804.

At S804, the amplifier circuit is powered up, as described above withrespect to FIG. 5, and operation of the process continues at S806

At S806, the control leads of Vdsat current source 216 related to thegeneration of a Vdsat current based on the IDAC current produced by IDACcurrent source 214, as described above with respect to FIG. 3, are setto an initial configuration selected by the technician based onpredetermined data, and operation of the process continues at S808.

At S808, if Vdsat current source 216 is configured to monitor thepoly-current through trim resistor 106, as described above with respectto FIG. 3, operation of the process continues at S810; otherwise,operation of the process continues at S812.

At S810, the control leads of Vdsat current source 216 related to thegeneration of a Vdsat current based on the poly-current that passesthrough trim resistor 106, as described above with respect to FIG. 3,are set to an initial configuration selected by the technician based onpredetermined data, and operation of the process continues at S812.

At S812, Vdsat current source 216 monitors the IDAC current, andoptionally, the poly-current, over a range of operating conditions,e.g., changes in operating temperatures, and operation of the processcontinues at S814.

At S814, if the technician observes a variation of Gm and/or amplifiergain at either the output of first amplifier stage 404 or secondamplifier stage 408 that exceeds an acceptable limit over the observedoperating range, operation of the process continues at S816 (as shown inFIG. 9); otherwise, operation of the process continues at S822.

At S816, the technician changes one or more of the Vdsat IDAC currentbased contribution to a first Vdsat current flow, e.g., throughtransistor 318 shown in FIG. 3, the Vdsat IDAC current basedcontribution to a second Vdsat current flow, e.g., through transistor324 shown in FIG. 3, the baseline IDAC current and/or the IDAC responsecurrent for one or more selected IDAC sensors for one or more selectedoperational conditions, e.g., at one or more temperatures, and operationof the process continues at S818.

At S818, if Vdsat current source 216 is configured to monitor thepoly-current through trim resistor 106, as described above with respectto FIG. 3, operation of the process continues at S820; otherwise,operation of the process continues at S812.

At S820, the technician changes one or more of the Vdsat poly-currentbased contribution to the first Vdsat current flow, and/or the Vdsatpoly-current based contribution to the second Vdsat current flow, andoperation of the process continues at S812.

At S822, if the technician determines that the magnitude of the appliedVdsat current and the magnitude of the applied IDAC current are withinestablished efficiency thresholds over the range of operationalconditions, e.g., the range of operational temperatures, operation ofthe process continues at S826; otherwise, operation of the processcontinues at S824.

At S824, the technician increments or decrements the trim resistor valueassigned to trim resistor 106, and operation of the process continues atS812.

At S826, the technician stores the determined configuration parametersand/or physically configures the circuit based on the determinedparameters, and operation of the process terminates at S828.

FIG. 10 is a schematic diagram of an example embodiment of an amplifierbiasing circuit that produces a constant Gm biasing signal for shortchannel amplifiers, as described above with respect to FIG. 4. As shownin FIG. 1, biasing circuit 1000 can include a first cascode transistor1002, a second cascade transistor 1004 that is substantially identicalto the first cascode transistor 1002, a trim resistor 1006, a mirrordevice transistor 1010, an operational amplifier 1012, a firststartup/protection diode transistor 1030, a second startup/protectiondiode transistor 1032, and a transistor 1034. First cascode transistor1002 includes a gate terminal, a source terminal and a drain terminal.Second cascode transistor 1004 includes a gate terminal, a sourceterminal and a drain terminal. Trim resistor 1006 includes a currentpath first terminal, a current path second terminal and a trim controlterminal 1008. Mirror device transistor 1010 includes a gate terminal, asource terminal and a drain terminal. Operational amplifier 1012includes a non-inverting input terminal, an inverting input terminal andan output terminal. First startup/protection diode transistor 1030includes a gate terminal, a source terminal and a drain terminal. Secondstartup/protection diode transistor 1032 includes a gate terminal, asource terminal and a drain terminal. Transistor 1034 includes a gateterminal, a source terminal and a drain terminal. In one exampleembodiment, the channel length of the gate channel of transistor 1034can be approximately the same as the channel length of the gate channelof mirror device transistor 1010; however, the width of the gate channelof transistor 1034 can be greater, e.g., 4 times greater, than the widthof the gate channel of mirror device transistor 110. As a result, thewidth-to-length ratio of transistor 1034 can be approximately 4 timesthe width-to-length ratio of mirror device transistor 1010, for reasonsdescribed below with respect to equation 1 through equation 6. Further,as described below with respect to FIG. 11, in one example embodiment,the equivalent width of the gate channel of transistor 1034 can beconfigurable, thereby allowing the Gm of biasing circuit 1000 to beadjusted.

As further shown in FIG. 10, the drain terminal of cascode transistor1002 is connected to HIGH voltage source V_(DD), the source terminal ofcascode transistor 1002 is connected to node 1018, and the gate terminalof cascode transistor 1002 is connected to the gate terminal of cascodetransistor 1004 at node 1020. The drain terminal of cascode transistor1004 is connected to HIGH voltage source V_(DD), and the source terminalof cascode transistor 1004 is connected to node 1022. The drain terminaland gate terminal of first startup/protection diode transistor 1030 isconnected to node 1018, and the source terminal of firststartup/protection diode transistor 1030 is connected to the sourceterminal of second startup/protection diode transistor 1032 at node1024. The drain terminal and gate terminal of second startup/protectiondiode transistor 1032 is connected to node 1022. The drain of mirrortransistor device 1010 is connected to node 1018, the source terminal ofmirror transistor device 1010 is connected to LOW voltage source V_(SS),and the gate terminal of mirror transistor device 1010 is connected tothe gate terminal of transistor 1034 at node 1024. The drain terminal oftransistor 1034 is connected to node 1022, and the source terminal oftransistor 1034 is connected to the current path first terminal of trimresistor 1006. The current path second terminal of trim resistor 1006 isconnected to LOW voltage source V_(SS). The first input node ofoperational amplifier 1012 is connected to node 1018, the second inputnode of operational amplifier 1012 is connected to node 1022, and theoutput node of operational amplifier 1012 is connected to node 1024.

Startup of biasing circuit 1000 is described in detail below withrespect to FIG. 12. It is noted, however, that startup/protection diodetransistor 1030 and startup/protection diode transistor 1032 play a rolein the startup of biasing circuit 1000. Startup/protection diodetransistor 1030 and startup/protection diode transistor 1032 prevent thevoltage at node 1024 from dropping to zero, which would prevent mirrortransistor device 1010 and drain terminal of transistor 1034 frompassing any current, and the voltage at node 1018 and node 1022 would beset at equal, full HIGH values. Since the voltage at node 1018 and node1022 would be equal, operational amplifier 1012 would be deactivated.Therefore, startup/protection diode transistor 1030 andstartup/protection diode transistor 1032 prevent such a state fromoccurring. If the output of operational amplifier 1012 is LOW, therespective diodes turn on and pull up the operational amplifier outputat node 1024. Further, if the voltage at node 1024 is high, mirrortransistor device 1010 will sink a large current, thereby pulling thenon-inverted input terminal of operational amplifier 1012 low, andtransistor 1034 will sink a limited current so that the inverted inputterminal of operational amplifier 1012 is not as low as the non-invertedinput terminal of operational amplifier 1012, thereby causingoperational amplifier 1012 to move away from such a high output state.

FIG. 11 is a schematic diagram of the amplifier biasing circuitdescribed above with respect to FIG. 10, in which additional details areshown. Features described above with respect to FIG. 10 are identifiedwith corresponding numeric labels. For example, first cascode transistor1002, a second cascode transistor 1004, a trim resistor 1006, a mirrordevice transistor 1010, an operational amplifier 1012, a firststartup/protection diode transistor 1030, a second startup/protectiondiode transistor 1032, and a transistor 1034 are depicted in a mannersimilar to that shown in FIG. 10, and therefore, are not furtherdescribed. However, as shown in FIG. 11, biasing circuit 1000 caninclude additional features such as a power-down switch 1106, a cascadegate storage capacitor 1108, and an optional tuning circuit 1102.

Power-down circuit 1106 allows the gates of mirror device transistor1010 and transistor 1034 to pulled LOW, as needed, to support circuitconfiguration and/or circuit power-down operations. Cascode gate storagecapacitor 1108 is used during operation to maintain a gate voltageestablished across the gates of cascode transistor 1002 and cascodetransistor 1004.

Optional tuning circuit 1102 is used to increase the effective gatewidth, i.e., reduce the effective resistance of transistor 1034, asneeded, to support various circuit configuration requirements. Forexample, as shown in FIG. 11, transistor 1122 of optional tuning circuit1102 is configured in parallel with transistor 1034. When OPEN,transistor 1122 will not pass any current or have any effect on biascircuit 1000. However, if transistor 1122 is fully CLOSED, additionalcurrent is able to pass from node 1022 to trim resistor 1006 in parallelwith transistor 1034. This has the effect of increasing the effectivewidth of transistor 1034.

As shown in FIG. 11, if transistor 1118 is CLOSED, the gate oftransistor 1122 will be connected to LOW voltage source, V_(SS), andtransistor 1122 will be held OPEN, and, therefore will not affectoperation of circuit 1000. However, if 1118 is OPENED, control signalsapplied to one or more of the gates of transistor 1110 or transistor1112 allow the gate of transistor 1122 to be connected to the gate oftransistor 1034 and may be controlled to operate in parallel withtransistor 1034, the current through transistor 1122 based on themagnitude of the control signals applied to transistor 1110 ortransistor 1112. Similarly, if transistor 1120 is CLOSED, the gate oftransistor 1124 will be connected to LOW voltage source, V_(SS), andtransistor 1124 will be held OPEN, and, therefore will not affectoperation of circuit 1000. However, if 1120 is OPENED, control signalsapplied to one or more of the gates of transistor 1114 or transistor1116 allow the gate of transistor 1124 to be connected to the gate oftransistor 1034 and may be controlled to operate in parallel withtransistor 1034, the current through transistor 1124 based on themagnitude of the control signals applied to transistor 1114 ortransistor 1116. In this manner, optional tuning circuit 1102 can beused to adjust the effective channel width of transistor 1034.

Bias circuit 1000 is a constant Gm bias circuit in which the Gm of thecircuit is set by the resistance value of trim resistor 1006 and theratio of the channel width-to-length ratios of mirror device transistor1010 and transistor 1034. The resistance of trim resistor 1006 may beconfigured with trim control terminal 1008, and the channel width oftransistor 1034 may be configured with optional tuning circuit 1102 toadjust the Gm of the circuit. To avoid current mismatches, or randomoffset, due to short channel effects, bias circuit 1000 maintains thesame drain-to-source voltage, Vds, across mirror device transistor 1010as the drain-to-source voltage, Vds, of the gain device of the amplifierstage receiving a bias signal from bias circuit 1000. Although the Gm ofbias circuit 1000 can be controlled in such a manner, bias circuit 1000is unable to further adjust the bias signal generated in response toprocess and temperature changes. If a single-stage amplifier circuitrequires the ability to adjust the bias signal in response to processand temperature changes, bias circuit 100 may be use in place of biascircuit 1000. If a multi-stage-stage amplifier circuit requires theability to adjust the gain of the multistage amplifier in response toprocess and temperature changes, bias circuit 100 may be used to providea biasing signal to one of the amplifier stages in the multi-stageamplifier, as described above with respect to FIG. 4, and bias circuit100 maybe adjusted to over compensate for the amplifier stage receivinga biasing signal from bias circuit 1000.

FIG. 12 a flow-chart of a process for starting up an example embodimentof the amplifier biasing circuit shown in FIG. 10 and FIG. 11. As shownin FIG. 12, operation of the process begins at S1202 and proceeds toS1204.

At S1204, power is supplied to bias circuit 1000, and operation of theprocess continues at S1206.

At S1206, voltage differences at node 1018 and 1022, as a result ofpowering up the circuit activate operational amplifier 1012, generate anoutput voltage at node 1024, and operation of the process continues atS1208.

At S1208, initial output voltage from operational amplifier 1012 closestransistor 1034 and establishes a gate-source voltage across the gate ofcascode transistor 1004, and operation of the process continues atS1210.

At S1210, the voltage established across the gate of cascode transistor1004 at S1208, results in a voltage at node 1020 and establishes agate-source voltage across the gate of cascode transistor 1002, therebyclosing cascade transistor 1002, and operation of the process continuesat S1212.

At S1212, a voltage is established at node 1022 on the inverted inputnode of operational amplifier 1012 as a result of a current passingthrough closed cascode transistor 1004, closed transistor 1034 and trimresistor 1006, and operation of the process continues at S1214.

At S1214, the voltage difference between the voltages applied at theinput nodes of operational amplifier 1012 fully activates operationalamplifier 1012, and operation of the process continues at S1216.

At S1216, mirror device transistor 1010 is closed by the voltagegenerated at the output node of operational amplifier 1012 at node 1024and operation of the process continues at S1218.

At S1218, a current is allowed to pass through mirror device 1010 andcascode transistor 1002, and operation of the process continues atS1220.

At S1220, if operational amplifier 1012 determines that a differencebetween the voltage at node 1018 and the voltage at node 1022 is zero,operation of the process terminates at S1224; otherwise, operation ofthe process continues at S1222.

At S1222, operational amplifier 1012 adjusts the output of operationalamplifier 1012 at node 1024 based on the difference between the voltagesapplied to the two respective input nodes of operational amplifier 1012at node 1018 and at node 1022, and operation of the process continues atS1220.

In the process described above, two feed back loops are established thatcontrol operation of bias circuit 1000. A first feedback loop isestablished between node 1024 and node 1018 that includes operationalamplifier 1012 and mirror device transistor 1010. A second feedback loopis established between node 1024 and node 1022 that includes operationalamplifier 1012 and transistor 1034. In response to any change in thevoltages at node 1018 and node 1022, operational amplifier 1012continues to adjust the voltage at node 1024, until the voltage at node1018 and node 1022 are again equalized via the respective feedbackloops. Once the difference between the input voltage at node 1018 andthe input voltage at node 1022 is zero, the bias circuit initializationprocess is complete.

In bias circuit 1000, the current density through mirror devicetransistor 1010 is given by equation 1, below.I ₁ =k/2*W ₁ /L ₁*(Vg ₁ −Vt ₁)²  EQ. 1

-   -   Where I₁ is the current density through the transistor;        -   k is the charge-carrier effective mobility of the transistor            device;        -   W₁ is the width of the transistor's channel;        -   L₁ is the length of the transistor's channel;        -   Vg₁ is the gate-to-source voltage of the transistor; and        -   Vt₁ is the gate threshold voltage of the transistor.

The current density through transistor 1034 is given by equation 2,below.I ₂ =k/2*W ₂ /L ₂*(Vg ₂ −IR−Vt ₂)²  EQ. 2

-   -   Where I₂ is the current density through the transistor;        -   k is the charge-carrier effective mobility of the transistor            device;        -   W₂ is the width of the transistor's channel;        -   L₂ is the length of the transistor's channel;        -   Vg₂ is the gate-to-source voltage of the transistor;        -   Vt₂ is the gate threshold voltage of the transistor; and        -   IR is the voltage drop across trim resistor 1006.

The Gm of transistor 1010 (Gm₁) is given by equation 3, below.Gm ₁=(2/R)*(1−1/sqrt(n))  EQ. 3

-   -   Where R is the resistance value of trim resistor 1006; and

$\begin{matrix}{n = \frac{\frac{W_{2}}{L_{2}}}{\frac{W_{1}}{L_{1}}}} & {{EQ}.\mspace{14mu} 4}\end{matrix}$

-   -   Where W₂ is the channel width of transistor 1034;        -   L₂ is the channel length of transistor 1034;        -   W₁ is the channel width of transistor 1010; and        -   L₁ is the channel length of transistor 1010.

The Gm of transistor 1034 (Gm₂) is given by equation 4, below.Gm ₂=(1/R)*(1−1/(2*sqrt(n)−1))  EQ. 5

-   -   Where n is defined by equation 4.

The ratio of the Gm of transistor 1010 to the Gm of transistor 1034 isgiven by equation 6, below.Gm ₁ /Gm ₂=2−1/sqrt(n)  EQ. 6

-   -   Where n is defined by equation 4.

Based on equation 6, Gm₁/Gm₂ is greater than 1 for n>1, so negativefeedback stronger than positive feedback. Further, Gm₁/Gm₂ approaches 2as n increases. For example, for n=4: mirror Gm=1/R and ratio=1.5.Therefore, in order to configure bias circuit 1000 with a constant Gm of1/R, transistor 1034 should be configured with a transistor channelwidth-to-length ratio that is 4-times the transistor channelwidth-to-length ratio of mirror device transistor 1010.

FIG. 13 is a flow-chart of a manual process that may be performed by atechnician to determine an appropriate resistor value for trim resistor1006, and an appropriate equivalent gate width for transistor 1034,within a specific implementation of bias circuit 1000, e.g., animplementation of bias circuit 1000 within a two-stage amplifier, suchas multi-stage amplifier 400 described above with respect to FIG. 4 iswhich bias circuit 1000 provides a bias signal to second amplifier stage408. Once the trim value and equivalent gate width are determined usingthe process described in FIG. 13, the same trim value and equivalentgate width settings may be used to configure similarly configuredcircuits. As shown in FIG. 13, operation of the process begins at S1302and proceeds to S1304.

At S1304, bias circuit 1000 is started as described above with respectto FIG. 12, and operation of the process continues at S1306.

At S1306, bias circuit 1000 is allowed to stabilize, and operation ofthe process continues at S1308.

At S1308, a technician measures selected operational parameters withinamplifier circuit 400 and/or within bias circuit 1000, e.g., voltagelevels, current levels, gain, Gm, etc., over a range of operationalconditions, e.g., a range of operating temperatures, and operation ofthe process continues at S1310.

At S1310, if a technician determines that performance and powerconsumption are acceptable, operation of the process continues at S1314;otherwise, operation of the process continues at S1312.

At S1312, the technician increments or decrements the resistor value,i.e., trims the resistor value, assigned to trim resistor 1006 and/oradjusts the effective gate width assigned to transistor 1034 usingoptional tuning circuit 1102, and operation of the process continues atS1306.

At S1314, the technician permanently assigns the determined resistorvalue to trim resistor 1006 and the determined gate width to transistor1034, and operation of the process terminates at S1316.

For purposes of explanation, in the above description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe described amplifier biasing circuit that reduces gain variation inshort channel amplifiers, the described amplifier biasing circuit thatproduces a constant Gm biasing signal for short channel amplifiers, andthe described multistage amplifier that uses example embodiments of bothtypes of biasing circuits. It will be apparent, however, to one skilledin the art that the described bias circuits and amplifier circuits maybe practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form inorder to avoid obscuring the features of the described amplifier biasingcircuit.

While the described amplifier biasing circuit that reduces gainvariation in short channel amplifiers, the described amplifier biasingcircuit that produces a constant Gm biasing signal for short channelamplifiers, and the described multistage amplifier that uses exampleembodiments of both types of biasing circuits have been described inconjunction with the specific embodiments thereof, it is evident thatmany alternatives, modifications, and variations will be apparent tothose skilled in the art. Accordingly, embodiments of the describedlevel bias circuits and amplifier circuits as set forth herein areintended to be illustrative, not limiting. There are changes that may bemade without departing from the scope of the invention.

What is claimed is:
 1. A bias signal circuit, comprising: a cascodetransistor configured to source a constant current; a constant currentsource configured to supply the constant current, the constant currentsource being connected in series with the cascode transistor; anoperational amplifier configured to counteract changes in a gain of thebias signal circuit at different operating temperatures for reducingvariations in the gain, a first input of the operational amplifierconnected to a drain terminal of a mirror device transistor; and avariable resistor configured to control a resistance, the variableresistor being connected in series with the mirror device transistor. 2.The bias signal circuit of claim 1, wherein a second input of theoperational amplifier is connected between the cascode transistor andthe constant current source, and an output of the operational amplifieris connected to a gate terminal of the mirror device transistor.
 3. Thebias signal circuit of claim 1, wherein the cascode transistor is afirst cascode transistor, further comprising: a second cascodetransistor having a second gate connected with a first gate of the firstcascode transistor, the second cascode transistor connected in serieswith the mirror device transistor.
 4. The bias signal circuit of claim3, wherein the operational amplifier is further configured to adjust sothat a gate-to-source voltage of the first cascode transistor plus avoltage drop across the variable resistor increases by a same amount asa gate-to-source voltage of the second cascode transistor.
 5. The biassignal circuit of claim 4, wherein the operational amplifier is furtherconfigured to increase a bias current flowing through the variableresistor by increasing a gate voltage of the mirror device transistor.6. The bias signal circuit of claim 3, wherein the variable resistor isconnected in series between the second cascode transistor and the mirrordevice transistor, the first input of the operational amplifier isconnected between the variable resistor and the mirror devicetransistor.
 7. The bias signal circuit of claim 3, wherein the secondcascode transistor has a second channel width-to-length ratio that isgreater than a first channel width-to-length ratio of the first cascodetransistor.
 8. The bias signal circuit of claim 7, wherein the secondchannel width-to-length ratio is approximately four times the firstchannel width-to-length ratio.
 9. The bias signal circuit of claim 1,further comprising: a variable current source configured to apply one ofa current source and a current sink at the drain terminal of the mirrordevice transistor.
 10. The bias signal circuit of claim 9, the variablecurrent source further comprising: a sensor controlled current sourceconfigured to determine a magnitude of at least a portion of a generatedcurrent based on an input signal received from a sensor.
 11. A method ofreducing gain variations of a bias signal circuit, comprising: sourcinga constant current by a cascode transistor; supplying the constantcurrent by a constant current source being connected in series with thecascode transistor; counteracting changes, by an operational amplifier,in a gain of the bias signal circuit at different operating temperaturesfor reducing variations in the gain, a first input of the operationalamplifier connected to a drain terminal of a mirror device transistor;and controlling, by a variable resistor, a resistance, the variableresistor being connected in series with the mirror device transistor.12. The method of claim 11, further comprising: receiving a second inputof the operational amplifier, the second input being connected betweenthe cascode transistor and the constant current source; and generatingan output of the operational amplifier, the output being connected to agate terminal of the mirror device transistor.
 13. The method of claim11, wherein the cascode transistor is a first cascode transistor,further comprising: coupling a second gate of a second cascodetransistor with a first gate of the first cascode transistor, the secondcascode transistor being connected in series with the mirror devicetransistor.
 14. The method of claim 13, further comprising: adjustingthe operational amplifier so that a gate-to-source voltage of the firstcascode transistor plus a voltage drop across the variable resistorincreases by a same amount as a gate-to-source voltage of the secondcascode transistor.
 15. The method of claim 14, further comprising:increasing, by the operational amplifier, a bias current flowing throughthe variable resistor by increasing a gate voltage of the mirror devicetransistor.
 16. The method of claim 13, further comprising: coupling thevariable resistor in series between the second cascode transistor andthe mirror device transistor, the first input of the operationalamplifier being connected between the variable resistor and the mirrordevice transistor.
 17. The method of claim 13, wherein the secondcascode transistor has a second channel width-to-length ratio that isgreater than a first channel width-to-length ratio of the first cascodetransistor.
 18. The method of claim 17, wherein the second channelwidth-to-length ratio is approximately four times the first channelwidth-to-length ratio.
 19. The method of claim 11, further comprising:applying one of a current source and a current sink at the drainterminal of the mirror device transistor by a variable current source.20. The method of claim 19, the variable current source furthercomprising: determining a magnitude of at least a portion of a generatedcurrent based on an input signal received from a sensor by a sensorcontrolled current source.